Jetting Anyone?

[CoalPilot]

Here is info I gathered from various sources. I believe this all to be correct and to be the nuts and bolts of the jetting issue all in one place. What I don't go into is the exact proceedures of setting the carb, as each carb manufacturer has it's own specs and sizes. Even two bikes exactly the same in every way may have to be jetted completely different. Oftentimes, guys will add-on every performance product available but they don't all play nicely together and become a nightmare to jet correctly. Cyl mods, reeds, pipes, etc all need special tuning. What if the pipe manufacturer suggests to go 2 sizes leaner on your main, but the pros who did your porting say to go 2 sizes richer??? Everything is trial and error when trying to get aftermarket equipment to work well together. Sometimes what you gain from the new fangled reeds you just installed, will be taken away by the mismatched pipe your running. and say, "What the heck, I don't see any gains!" If I am wrong about any of this or if you have anything to add, feel free.

A few things to check that affect jetting. Go through the list and fix any problems you find before attempting to adjust you carb. Beleive me it will save you a lot of time a frustration in getting your bike running right. DON"T ASSUME ANYTHING!!!! Check it anyway you may find something went bad, was misinstalled, overlooked, etc.. Just because you had something "professionally done" does not mean thay they could have forgotten or missed something.

I once had a local dealer who I never used before, do a bore for me. Bike would always have a problem somewhere and could not get it right. I dismissed the bore job because it was "professionally done"...WRONG!!! When I decided to finally pull the cyl. The rings never seated correctly and instead of a nice smooth shiny bore, I had stripes!!!! The machinists equip was out of alignment. They did make good on it an replaced the cyl. with a brand new one because it could not be bored again. All I'm saying is don't leave anything to chance. Like the carpenters always say, measure twice, cut once. Thery just don't make any good 2x4 streachers anymore. Ha! Ha! Ha!

* Air Leaks - Will make the bike sluggish and may produce a pinging sound. This occurs when the mixture becomes lean from the air leak.

* Right side crank seal - Leakage here will cause your spark plug to carbon foul and make you jetting seem too rich.

* Coolant leaks - Can cause misfire or popping sounds at the exhaust. This will cause a loss in peak HP and make it run as if it were too rich.

* Reeds - Broken, cracked, or non-sealing reeds will make the bike hard to start and have loss of torque.

* Silencer packing - Blown out or spoog soaked packing will make the bike run flat at high RPM's.

* Power Valve - Carbon siezed or gummed up exhaust valves make the bike run flat at low rpm's and makes your slow-speed jetting seem too lean.

* Coil - Weak coil will cause high speed misfire. make sure coil is properly grounded. Hot glue, silicone, or rubber dip make good sealing agents around mounting surfaces and bolts keeping moisture out to prevent rust and corrosion.

* Fuel inlet needle and seat should be replaced every other year. A worn one will cause fuel to travel up the pilot jet (slow jet) and into the engine making the jetting seem too rich.

*Float level - Too LOW a float level makes the jetting seem too lean and performance sluggish. Too HIGH a flost level makes the jetting seem too rich and may even start spewing gas from vent tubes and overflow.

Surging - Is mainly caused by a too lean condition especially in the pilot jet. But first check for air leaks at carb joint boot, reed block, cyl. base, crankcase, etc. before going a size larger on the pilot.

There are 3 circuits that control air:
1. Air Screw
2. Slide
3. Air Jet

There are 4 circuits that control Fuel:
1. Pilot Jet also known as Slow Jet
2. Needle Jet
3. Jet Needle
4. Main Jet

From throttle closed to 1/8 open is the idle circuit and is controlled by the Pilot Jet and Air Screw.
From 1/8 to 1/4 throttle transitioning into midrange is controlled by Pilot Jet, Air Screw, and Slide.
From 1/4 to 1/2 throttle midrange is controlled by Slide and Jet Needle.
From 1/2 to Full open throttle (WOT) is controlled by Jet Needle, Needle Jet, Main Jet, and Air Jet.

Real world test.....In 2nd or 3rd gear, roll on the throttle slowly from 1/4 to 1/2 open.
If it is slow to respond and bogs (Booooowah sound), then the carb jetting in that area is too Lean. If it makes excessive smoke and a crackling sound or seems to be running like the choke is on, The carb setting is too Rich. Make adjustments to slide and Jet needle.

Performing the same test in 3rd or 4th gear from 1/2 to Full open (WOT) will tell you about the top end jetting wether too Lean or too Rich. The areas to be adjusted are Jet needle, Main Jet, and air jet. When set correctly, doing a throttle chop test should result in a nice tan spark plug reading.

Premix ratios must always be consistant and variations will affect your jetting. It is better to have your carb jetting set correctly than to adjust your jetting through altering premix ratios. But if you must, here is the theory of how premix ratios affect carb jetting.....

First, the ratio (like 32:1) means 32 parts Gas to 1 part oil. Many get confused on this because they add the oil to the Gas and not Gas to the oil. They think, "How may parts of this oil do I add to my Gas?" I am not keen on the oil bottle marking graduations and do not feel they are accurate. Get yourself an oil ratio mixing cup/beaker or something that measures in "cc".

2 US Gallons equals 7,570.82 cc
2 1/2 US Gallons equals 9,463.52 cc
5 US Gallons equals 18,927.05 cc

If you take one of the above quantities, say like 7,570.82 cc (2 Gallons) and divide by your desired ratio (like 32:1), then 7,570.82 / 32 = 236.588 cc of oil to be added to the 2 Gallons of gas making your 32:1 ratrio.

Always go by the oil manufacturers mixing recommendation for their brand and NOT by what your owners manual tells you....unless you are using the owners manual recommended oil.

Many get confused on Lean or Rich mixture. They think LEAN means MORE GAS and LESS OIL. And RICH means MORE OIL and LESS GAS. The opposite is true.

30:1 = 30 parts Fuel to 1 part oil (less fuel) Leaner
40:1 = 40 parts Fuel to 1 part oil (more fuel) Richer

More Fuel makes the mixture RICHER, More Oil makes the mixture LEANER.

If you RICHEN the mixture (30:1 instead of 40:1) there will be more oil and less fuel which will LEAN the Carb air/fuel ratio jetting.

*You Richen the carb jetting by reducing the premix ratio (less oil)
*You Lean the carb jetting by increasing the premix ratio (more oil)

 

[Rcannon]

Nice post. Consider it saved and printed!

 

[ellandoh]

this is my favorite along w/EG's which everyone here should know

Spanky's jetting guide:

A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband. A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using. A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift.
Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting.
The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless.
Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best.
It's very important that you start with the pilot circuit. The reason is simple. The pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving.
Before you start to rejet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel. One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting.
Before you start the jet testing, install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect your jetting all across the throttle range.
Warm the bike completely, and shut it off.
As already stated, start with the pilot circuit. Turn the airscrew all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idleing. Turn the airscrew slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the airscrew for the best response.
Now is the time to determine if you have the correct pilot installed in your carb. The airscrew position determines this for you, making it very simple. If your airscrew is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet.
Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the airscrew for the fastest idle, it's time to tune the airscrew for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the airscrew slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn.
The airscrew is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the airscrew to compensate for changing outdoor temps and humidity. An airscrew setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.
Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong.
Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan.
Once you have a little bit of experience with jetting changes, and you start to learn the difference in feel between "rich" and "lean", you'll begin to learn, just from the sound of the exhaust and the feel of the power, not only if the bike is running rich or lean, but even which one of the carb circuits is the culprit.
The slide is also a tuning variable for jetting, but slides are very expensive, and few bikes need different slides, so we won't go into that here.
Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit. Two-strokes have an air screw that you screw in to make the jetting richer, and screw out to make the jetting leaner. Four-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer.